33.3.1.1 Special Characters in Regular Expressions

Here is a list of the characters that are special in a regular
expression.

‘.’ (Period)

is a special character that matches any single character except a newline.
Using concatenation, we can make regular expressions like ‘a.b’, which
matches any three-character string that begins with ‘a’ and ends with
‘b’.

‘*’

is not a construct by itself; it is a postfix operator that means to
match the preceding regular expression repetitively as many times as
possible. Thus, ‘o*’ matches any number of ‘o’s (including no
‘o’s).

‘*’ always applies to the smallest possible preceding
expression. Thus, ‘fo*’ has a repeating ‘o’, not a repeating
‘fo’. It matches ‘f’, ‘fo’, ‘foo’, and so on.

The matcher processes a ‘*’ construct by matching, immediately, as
many repetitions as can be found. Then it continues with the rest of
the pattern. If that fails, backtracking occurs, discarding some of the
matches of the ‘*’-modified construct in the hope that that will
make it possible to match the rest of the pattern. For example, in
matching ‘ca*ar’ against the string ‘caaar’, the ‘a*’
first tries to match all three ‘a’s; but the rest of the pattern is
‘ar’ and there is only ‘r’ left to match, so this try fails.
The next alternative is for ‘a*’ to match only two ‘a’s. With
this choice, the rest of the regexp matches successfully.

Warning: Nested repetition operators can run for an
indefinitely long time, if they lead to ambiguous matching. For
example, trying to match the regular expression ‘\(x+y*\)*a’
against the string ‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz’ could
take hours before it ultimately fails. Emacs must try each way of
grouping the ‘x’s before concluding that none of them can work.
Even worse, ‘\(x*\)*’ can match the null string in infinitely
many ways, so it causes an infinite loop. To avoid these problems,
check nested repetitions carefully, to make sure that they do not
cause combinatorial explosions in backtracking.

‘+’

is a postfix operator, similar to ‘*’ except that it must match
the preceding expression at least once. So, for example, ‘ca+r’
matches the strings ‘car’ and ‘caaaar’ but not the string
‘cr’, whereas ‘ca*r’ matches all three strings.

‘?’

is a postfix operator, similar to ‘*’ except that it must match the
preceding expression either once or not at all. For example,
‘ca?r’ matches ‘car’ or ‘cr’; nothing else.

‘*?’, ‘+?’, ‘??’

These are non-greedy variants of the operators ‘*’, ‘+’
and ‘?’. Where those operators match the largest possible
substring (consistent with matching the entire containing expression),
the non-greedy variants match the smallest possible substring
(consistent with matching the entire containing expression).

For example, the regular expression ‘c[ad]*a’ when applied to the
string ‘cdaaada’ matches the whole string; but the regular
expression ‘c[ad]*?a’, applied to that same string, matches just
‘cda’. (The smallest possible match here for ‘[ad]*?’ that
permits the whole expression to match is ‘d’.)

‘[ … ]’

is a character alternative, which begins with ‘[’ and is
terminated by ‘]’. In the simplest case, the characters between
the two brackets are what this character alternative can match.

Thus, ‘[ad]’ matches either one ‘a’ or one ‘d’, and
‘[ad]*’ matches any string composed of just ‘a’s and ‘d’s
(including the empty string). It follows that ‘c[ad]*r’
matches ‘cr’, ‘car’, ‘cdr’, ‘caddaar’, etc.

You can also include character ranges in a character alternative, by
writing the starting and ending characters with a ‘-’ between them.
Thus, ‘[a-z]’ matches any lower-case ASCII letter.
Ranges may be intermixed freely with individual characters, as in
‘[a-z$%.]’, which matches any lower case ASCII letter
or ‘$’, ‘%’ or period.

If case-fold-search is non-nil, ‘[a-z]’ also
matches upper-case letters. Note that a range like ‘[a-z]’ is
not affected by the locale’s collation sequence, it always represents
a sequence in ASCII order.

Note also that the usual regexp special characters are not special inside a
character alternative. A completely different set of characters is
special inside character alternatives: ‘]’, ‘-’ and ‘^’.

To include a ‘]’ in a character alternative, you must make it the
first character. For example, ‘[]a]’ matches ‘]’ or ‘a’.
To include a ‘-’, write ‘-’ as the first or last character of
the character alternative, or put it after a range. Thus, ‘[]-]’
matches both ‘]’ and ‘-’. (As explained below, you cannot
use ‘\]’ to include a ‘]’ inside a character alternative,
since ‘\’ is not special there.)

To include ‘^’ in a character alternative, put it anywhere but at
the beginning.

If a range starts with a unibyte character c and ends with a
multibyte character c2, the range is divided into two parts: one
spans the unibyte characters ‘c..?\377’, the other the
multibyte characters ‘c1..c2’, where c1 is the
first character of the charset to which c2 belongs.

A character alternative can also specify named character classes
(see Char Classes). This is a POSIX feature. For example,
‘[[:ascii:]]’ matches any ASCII character.
Using a character class is equivalent to mentioning each of the
characters in that class; but the latter is not feasible in practice,
since some classes include thousands of different characters.

‘[^ … ]’

‘[^’ begins a complemented character alternative. This
matches any character except the ones specified. Thus,
‘[^a-z0-9A-Z]’ matches all characters except letters and
digits.

‘^’ is not special in a character alternative unless it is the first
character. The character following the ‘^’ is treated as if it
were first (in other words, ‘-’ and ‘]’ are not special there).

A complemented character alternative can match a newline, unless newline is
mentioned as one of the characters not to match. This is in contrast to
the handling of regexps in programs such as grep.

You can specify named character classes, just like in character
alternatives. For instance, ‘[^[:ascii:]]’ matches any
non-ASCII character. See Char Classes.

‘^’

When matching a buffer, ‘^’ matches the empty string, but only at the
beginning of a line in the text being matched (or the beginning of the
accessible portion of the buffer). Otherwise it fails to match
anything. Thus, ‘^foo’ matches a ‘foo’ that occurs at the
beginning of a line.

When matching a string instead of a buffer, ‘^’ matches at the
beginning of the string or after a newline character.

For historical compatibility reasons, ‘^’ can be used only at the
beginning of the regular expression, or after ‘\(’, ‘\(?:’
or ‘\|’.

‘$’

is similar to ‘^’ but matches only at the end of a line (or the
end of the accessible portion of the buffer). Thus, ‘x+$’
matches a string of one ‘x’ or more at the end of a line.

When matching a string instead of a buffer, ‘$’ matches at the end
of the string or before a newline character.

For historical compatibility reasons, ‘$’ can be used only at the
end of the regular expression, or before ‘\)’ or ‘\|’.

‘\’

has two functions: it quotes the special characters (including
‘\’), and it introduces additional special constructs.

Because ‘\’ quotes special characters, ‘\$’ is a regular
expression that matches only ‘$’, and ‘\[’ is a regular
expression that matches only ‘[’, and so on.

Note that ‘\’ also has special meaning in the read syntax of Lisp
strings (see String Type), and must be quoted with ‘\’. For
example, the regular expression that matches the ‘\’ character is
‘\\’. To write a Lisp string that contains the characters
‘\\’, Lisp syntax requires you to quote each ‘\’ with another
‘\’. Therefore, the read syntax for a regular expression matching
‘\’ is "\\\\".

Please note: For historical compatibility, special characters
are treated as ordinary ones if they are in contexts where their special
meanings make no sense. For example, ‘*foo’ treats ‘*’ as
ordinary since there is no preceding expression on which the ‘*’
can act. It is poor practice to depend on this behavior; quote the
special character anyway, regardless of where it appears.

As a ‘\’ is not special inside a character alternative, it can
never remove the special meaning of ‘-’ or ‘]’. So you
should not quote these characters when they have no special meaning
either. This would not clarify anything, since backslashes can
legitimately precede these characters where they have special
meaning, as in ‘[^\]’ ("[^\\]" for Lisp string syntax),
which matches any single character except a backslash.

In practice, most ‘]’ that occur in regular expressions close a
character alternative and hence are special. However, occasionally a
regular expression may try to match a complex pattern of literal
‘[’ and ‘]’. In such situations, it sometimes may be
necessary to carefully parse the regexp from the start to determine
which square brackets enclose a character alternative. For example,
‘[^][]]’ consists of the complemented character alternative
‘[^][]’ (which matches any single character that is not a square
bracket), followed by a literal ‘]’.

The exact rules are that at the beginning of a regexp, ‘[’ is
special and ‘]’ not. This lasts until the first unquoted
‘[’, after which we are in a character alternative; ‘[’ is
no longer special (except when it starts a character class) but ‘]’
is special, unless it immediately follows the special ‘[’ or that
‘[’ followed by a ‘^’. This lasts until the next special
‘]’ that does not end a character class. This ends the character
alternative and restores the ordinary syntax of regular expressions;
an unquoted ‘[’ is special again and a ‘]’ not.